Extension of direct displacement-based design methodology for bridges to account for higher mode effects

An improvement is suggested to the direct displacement-based design (DDBD) procedure for bridges to account for higher mode effects, the key idea being not only the proper prediction of a target-displacement profile through the effective mode shape method (wherein all significant modes are considered), but also the proper definition of the corresponding peak structural response. The proposed methodology is then applied to an actual concrete bridge wherein the different pier heights and the unrestrained transverse displacement at the abutments result in an increased contribution of the second mode. A comparison between the extended and the standard' DDBD is conducted, while further issues such as the proper consideration of the degree of fixity at the pier's top and the effect of the deck's torsional stiffness are also investigated. The proposed methodology and resulting designs are evaluated using nonlinear response-history analysis for a number of spectrum-compatible motions. Unlike the standard' DDBD, the extended procedure adequately reproduced the target-displacement profile providing at the same time a good estimate of results regarding additional design quantities such as yield displacements, displacement ductilities, etc., closely matching the results of the more rigorous nonlinear response-history analysis. However, the need for additional iterations clearly indicates that practical application of the proposed procedure is feasible only if it is fully automated', that is, implemented in a software package. Copyright (c) 2012 John Wiley & Sons, Ltd.